Although I had arrived as early as December 2016 the last day of January 2017 was indeed my first day out for bird watching after I had come here for overwintering. I had been occupied by too many other things. But today I had to visit the Immigration Office near the airport and as this is more than half way between my home and the area below Songino Khairkhan Uul, I quickly drove down there and spent a couple of hours birding the berry eaters. This winter the trees carry a full load of berries and this did not only attract the more or less usual number of Bohemian Waxwings (c500) but also a higher than usual number of thrushes. Photographing the waxwings has become a kind of routine and I tried not to take too many photos of them. They had been featured frequently and hopefully also thoroughly enough (see here). Despite this I looked for an adult male with those red waxy plates on the rectrices but failed again in finding one. My search for the Holy Grail of the waxwings, Japanese Waxwing, was also fruitless so I need to come again. As always, the waxwings were very flighty and I surely did not check them all.On one hand, wintertime is arguably not the best time for studying thrushes: not much study material is around and the birds are not that social and flocks are rare. Instead they install small territories which are even defended against waxwings. On the other hand, if only a few birds are present, one can fully concentrate on details, much better so than during the peak migration times when hundreds of thrushes can be seen within a single flock.The “gulls” of the thrushes—by the means of causing ID headaches—is a quartet of taxa which interbreed: Red-throatedTurdus ruficollis (hereafter RTT) and Black-throated ThrushesT. atrogularis (BTT) are commonly subsumed under the name Dark-throated Thrush. The other pair consists of Naumann’sT. naumanni and Dusky ThrushT. eunomus, which also are sometimes regarded to belong to a single species, Dusky Thrush. The two species of the first mentioned pair are certainly the most common wintering thrushes, whereas of the latter pair only a very few Naumann’s and even fewer Dusky Thrushes have been recorded during winter time. Because hybridization is not only recorded within each species pair but also between members of different pairs, a full array of hybrids must be expected and very often the increasingly desperate observer ends up with the likewise often unanswerable question “Does the bird look like this because of intraspecific variation or because of some degree of gene flow between two (or more) taxa?”. This post will not answer this question, unfortunately, but it will hopefully show that the closer you look, the more disturbing details might come to light.I couldn’t take pictures of all individuals that I saw today and of most I photographed I would have liked to take more pictures, but this I never achieved.

3. Male Black-throated Thrush

4. The same male

5. Tail base details of the same male

6. The other side of the tail of the same male

Let’s start with this easy adult male BTT (pictures 3 to 6). No problem with this, or? Both, Lewington et al. (1991) as well as Clement & Hathway (2000) quote that BTT does not show any red or orangey tone to the feathers (apart from the underwing coverts, see picture 5, where such a displaced orange underwing feather shows on the upper side), but Glutz von Blotzheim & Bauer (1988) allow a faint rusty coloration on the outer vanes of all but the central pair of tail feathers. This can be seen in our first bird (pictures 5 and 6). Additionally there is weak orange coloration on the otherwise entirely white undertail coverts which is only visibly from a very short distance. In field this probably would go unnoticed.

7. Male Black-throated x Red-throated Thrush

A much cropped picture

8. The same bird, a bit closer

What about the next one (pictures 7 to 11)? Easy as well, eh? From the distance this adult male looks boldly black and white and could be mistaken for Common Magpie or even Rosy Starling by the beginner. Both, of course, would show all-black undertail coverts but in this bird they are white. Some darker spots, created by the darker bases of the undertail coverts shine through, though. Usually the darker parts of the undertail coverts are obscured by the large white tips resulting in a plain all-white look to this part. Luckily the male decided to move in closer and on the second picture the under tail is just white (picture 8, but see also pictures 10 & 11). All feathers of the throat and breast are black with a narrow white margin (which wears off quickly and during the breeding season these parts are very very dark). So it is just another straightforward bird? A clean adult male BTT? Not so fast please! We need to look closer.

9. The same male jumping down to the ground

10. Right side of this bird

11. And the left hand side as well

I got a blurry flight shot (picture 9) when the bird dashed down to the ground for a foraging session. Here, the dark bases of the under tail coverts can “clearly” be seen. On the longest coverts these bases are dark, maybe even black, but the shorter coverts have a kind of brownish bases. Obviously, the underside of the tail is not black. Towards the outer pair of tail feathers the brownish hue intensifies. Could these be already hints to some RTT influence? The upperside of the tail shows some orange red on the bases on both sides (pictures 10 & 11). Following Lewington et al. (1991) as well as Clement & Hathway (2000) these orange tail corners are the disqualifiers for being it a pure BTT. Whether the amount of orange shown by this bird would be just OK for being accepted as a pure BTT by Glutz von Blotzheim (1988) is impossible to know as the authors don’t provide an upper limit. So if these birds really are hybrids, a great deal of care needs to be executed to enable someone to pinpoint a true BTT. It would as quite interesting as it would be helpful to know whether red tail corners on adult male BTT or BTT-like hybrids are evenly distributed over the entire breeding range of BTT. Do they occur also in the western part of the BTT range which is far from the breeding range of RTT? If the proportion of red-corned males would increase from west (zero?) to the east (percentage unknown!) this would be a strong point for some genetically leakage of RTT DNA into the BTT population and hence would proof the assumption that any red on the tail safely identifies a hybrid. At present this is all entirely unknown and I am quite sure the above mentioned authors could NOT assure the genetic entity of the birds they had used to draw their respective conclusions.Haffer (1988) and also Clement & Hathway (2000) claim that intermediate birds (=hybrids) look quite similar to same age and sex BTT which implies that the dark coloration is dominant over the red. To proof this it would be necessary to breed these thrushes under controlled conditions, i.e. pairing up pure (DNA tested and with preferably pure parents) parents of either taxon and see how their offspring looks like. I do not think that anybody ever has undertaken such an effort.So isn’t it just a matter of detectability? Under the assumption that a pure BTT has no orange on the upperside, any orange (then a sign of hybridism), be it even very small, would be rather obvious and could easily be detected. Because of this easiness many observers are able to identify those individuals as hybrid. In case of a rather RTT-like hybrid the recognition wouldn’t be as easy. A slight reduction of the amount of orange on a RTT would be very difficult to detect, especially as some variation on the amount of dark on the tail feathers has been described. The darker end of this variation could actually indicate the introduction of BTT DNA into the stock of RTT (check pictures 12, 13 and 18 for some examples).

12. Young male Black-throated x Red-throated Thrush

13. The same bird from the other side

The next guy shows a bright reddish supercilium, rufous cheeks and lots of orange in the tail (picture 12). When seen head on (picture 13) the uneven length of the greater upperwing coverts illustrate that this is a 2cy thrush (the outer and shorter feathers being juvenile feather and the longer ones had been already moulted but this is not always as easy to see as in this individual). The solidly darkish breast makes it a male. To me, the breast feathers are too dark for a pure RTT but I am not so sure whether the amount of dark on all of the visible tail feathers is too much for a pure RTT. So is it a just a darker RTT or has some foreign DNA crept in? I don’t know. For the time being I booked this as a BTT x RTT hybrid.

14. Black-throated x Red-throated Thrush

Below Songino Khairkhan Uul

UB, January 2017

If the bird in picture 14 is a 2cy individual-and the white tips to the outer greater coverts which are also shorter than the two inner one suggest this—it should be a male (but some females may show a very distinct breast shield!). Its supercilium is suffused be red and the underside of the tail shows red as well so my conclusion is: BTT x RTT.

15. Black-throated x Red-throated Thrush

Below Songino Khairkhan Uul, UB, January 2017

16. The same bird from the side

17. And the same bird again in a heavily cropped image

Now (pictures 15 to 17) another female (?) thrush with a distinct supercilium and a large white throat area but a solid brownish and blackish breast shield. Already when see head-on (picture 15 and the heavily cropped picture 17) the breast looks neither good for a pure BTT nor for a pure RTT. It even has a reddish area on the neck sides and its tail show red, but not as much as I would expect in a pure RTT. This should be just another BTT x RTT hybrid.

18. Black-throated x Red-throated Thrush?

Below Songino Khairkhan Uul,

UB, January 2017

What about the bird in picture 18? It also doesn’t show lots of red in the tail and its breast looks almost red free. Is it within the variation of RTT or not? I identified this one as a BTT x RTT but I am totally unsure about this.It seems that there were almost no pure birds around that day. But I photographed at least two thrushes that can be safely identified as pure young female BTT. These are shown in the pictures 19 and 20. As can be seen in the latter there were some easy to identify thrushes around: Fieldfare.

19. Female Black-throated Thrush

Below Songino Khairkhan Uul, UB, January 2017

20. Female Black-throated Thrush, background: Fieldfare

Below Songino Khairkhan Uul, UB, January 2017

21. Fieldfare below Songino Khairkhan Uul

UB, January 2017

22. Fieldfare below Songino Khairkhan Uul

UB, January 2017

23. Fieldfare below Songino Khairkhan Uul

UB, January 2017

24. Fieldfare below Songino Khairkhan Uul

UB, January 2017

25. Putative Red-throated x Naumann’s Thrush

Below Songino Khairkhan Uul

UB, January 2017

But there was also this thrush (picture 25) of which I managed to get a single shot from the distance. If a thrush shows an obvious supercilium behind the eye alarm bells start ringing as I take this always as an indicator for Naumann’s or Dusky Thrush influence (if it is not a pure of one of those). The orange underside of the tail plus the almost plain pale orange undertail coverts are good for a Naumann’s Thrush but the entire upperparts point more towards RTT, especially because of the reddish spot on the neck side. The multiple rows of tiny dark-centered orange spots along the breast and down the flanks are unusual for either of these candidates. For me this looks like a RTT x Naumann’s hybrid.These are really headache-causing birds and any expertise intellectual input on this topic, be it on ageing, sexing or even general, would be very highly welcomed, especially if the captions would have to be corrected!Just for pleasure I include some Bohemian Waxwings. Can you find the only female and the bird, which cannot be aged?

31. Dark morning below Songino Khairkahn UulUB, January 2017Solution: The only female (almost white outer tips of the primaries, no waxy tips to the secondaries, a blurred lower border of the dark throat and a narrow yellow tail tip) can be seen in picture 29 and the bird in picture 27 cannot be aged because the upperside of the primaries is not visible.

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BIRDING MONGOLIA covers any topic concerning wild birds in Mongolia: conservation, migration, observations, identification, projects, events etc. Contributions on other aspects of Mongolia’s natural heritage, as well as information from regions bordering Mongolia, are also featured from time to time.

Axel Bräunlich

I have been birding in Mongolia since 1995. Birding Mongolia was originally set up in March 2007, providing information on the fabulous birdlife (especially migration) at my local patch in Khovd, near the Altai Mountains in the west of the country, where I lived from 2005 to 2007. The blog grew rapidly, with contributions coming from more and more observers, and covering other parts of Mongolia, too.

Andreas Buchheim

I have been visiting Mongolia since 2004. It all started with gull research trips, but I now come to Mongolia at least twice a year, staying for about 6 months annually since 2010, not only for the continuing gull research but now mainly to be with my Mongolian wife and our son and to go birding. As a quasi resident of Ulaanbaatar, I do most of my birding around the city during winter but, luckily, I can visit the even more fascinating rural regions of this rapidly changing country during the warmer seasons.

Contributions (observations, photos, news etc) are very welcome! Any material contributed will be acknowledged and the copyright will remain with the author(s).

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